1. Technical Field
The present invention relates generally to systems for processing compressed video data, and more particularly to a system and method for dividing MPEG video data into multiple streams of different priority.
2. Related Art
As the demand for the on-line delivery of video applications continues to grow and transmission bandwidth remains at a premium, the ability to provide highly efficient transmission schemes has become indispensable. One such scheme involves dividing MPEG coded video data into multiple streams, each having a different priority. This type of scheme, referred to herein as “multi-priority streams,” allows different portions of the video data to consume different amounts of bandwidth, depending on the importance of the particular portion.
One specific scenario where multi-priority streams are useful involves applications that utilize transmission channels susceptible to errors. In these cases, it is necessary to protect the data with additional error protection bits, which may for example provide a certain amount of redundancy. Unfortunately, the inclusion of error protection bits creates additional overhead in terms of bandwidth consumption. By dividing MPEG data into a plurality of streams of differing priorities, individual streams can be assigned different levels of error protection. Accordingly, high priority streams can receive a greater amount of error protection than lower priority streams. Thus, overhead can be balanced with the corresponding priority of the streams.
A second scenario where multi-priority streams are useful involves networks that offer multiple quality of service (QoS) options. In such a case, different levels of quality of service for transmitted data are made available by the underlying transport. Thus, in order to efficiently utilize such a system, it is necessary to divide the video data into streams of differing priority, and then send each stream over the connection offering the appropriate QoS.
Unfortunately, existing methods of creating multi-priority streams for MPEG video are limited. For example, one approach is to divide the bitstream into different priorities based on the type of frame. In particular, since for picture quality I frames are generally more important than P frames, and P frames are generally more important than B frames, I frames receive a highest priority, P frames receive a medium priority, and B frames receive a lowest priority. However, because the actual size of a typical I or P frame can be hundreds of thousands of bits, this type of prioritization provides only marginal results since only three priority levels can be implemented. Moreover, there can be a large amount of variation in terms of importance between two I frames, or two P frames, and such variations are not taken into account in the above scheme. Thus, a need exists to create a finer prioritization among and between frames.
A second approach for creating multi-priority streams for MPEG video is to create scalable video that contains a base layer and one or more enhancement layers. The base layer is assigned a higher priority than the enhancement layers. The drawback of this approach is the increased complexity required for encoding and decoding, and the need to transcode the already coded non-scalable bitstream into a scalable bitstream.
Accordingly, a system is required: (1) that can provide a finer prioritization than the I, P and B frame prioritization; and (2) that can directly process a coded bitstream without incurring the complexity of a system that utilizes scalable video.